The proliferation and popularity of mobile radio and telephony applications has led to market demand for communication systems with low cost, low power, and small form-factor radio-frequency (RF) transceivers. As a result, recent research has focused on providing monolithic transceivers using low-cost complementary metal-oxide semiconductor (CMOS) technology. One aspect of research efforts has focused on providing an RF transceiver within a single integrated circuit (IC). The integration of transceiver circuits is not a trivial problem, as it must take into account the requirements of the transceiver's circuitry and the communication standards governing the transceiver's operation.
From the perspective of the transceiver's circuitry, RF transceivers typically include sensitive components susceptible to noise and interference with one another and with external sources. Integrating the transceiver's circuitry into one integrated circuit may exacerbate interference among the various blocks of the transceiver's circuitry. Moreover, communication standards governing RF transceiver operation outline a set of requirements for noise, inter-modulation, blocking performance, output power, and spectral emission of the transceiver. Unfortunately, no technique for addressing all of the above issues in high-performance RF receivers or transceivers, for example, RF transceivers used in cellular and telephony applications, has been developed. A need therefore exists for techniques of partitioning and integrating RF receivers or transceivers that would provide low-cost, low form-factor RF transceivers for high-performance applications, for example, in cellular handsets.
A further aspect of RF apparatus, such as RF transceivers and transmitters, relates to the transmitter circuitry or transmit-path circuitry. Typical transmit circuitry includes a feedback loop (often a phase-locked loop, or PLL) that has a voltage-controlled oscillator (VCO) and a loop filter circuitry. In conventional transmitters and transceivers, the VCO circuitry and the loop filter circuitry constitute off-chip, off-the-shelf, discrete components. That arrangement, however, has several disadvantages. The external components require routing on-chip signals to those components and, conversely, routing signals from the discrete components to on-chip integrated circuitry. Consequently, noise sensitivity and susceptibility increases, while the effective operating frequency decreases. Furthermore, discrete components increase the overall system cost, complexity, power consumption, and form factor (e.g., board size, number of package pins). Worse yet, discrete components reduce the system's overall integration level, reliability, and speed or throughput.
In addition, conventional discrete VCOs typically have relatively large gains (i.e., a relatively small change in the VCO's control voltage results in a relatively large change in the frequency of the VCO's output signal). The large gain results in more sensitivity and susceptibility to noise. Thus, noise or spurious signals added to or coupled to the control voltage might corrupt the fidelity of the VCO by causing undesired variations in the frequency of the VCO's output signal or otherwise result in impurity of the output signal. As mentioned above, the conventional discrete VCO circuitry typically requires the user to route signals from the RF integrated circuitry to the discrete VCO circuitry, thus increasing the likelihood of corruption by noise and spurious signals and exacerbating the problems described above. A need therefore exists for integrated VCO circuitry (to reduce cost and/or size) within the transmit-path circuitry of RF apparatus, such as transceivers and transmitters.
Often, the user desires the transmit-path circuitry to operate in more than one band (i.e., it supports multi-band operation). Examples of various bands include GSM 850, GSM 900, DCS 1800, and PCS 1900. In conventional RF apparatus, operation in each additional band typically entails the provision of an additional discrete VCO circuitry. Thus, a multi-band RF apparatus may include several discrete VCO circuitries. Consequently, in conventional RF apparatus, the problems associated with discrete VCO circuitries described above compound as the number of VCO circuitries increases. A further need therefore exists for RF apparatus that provides multi-band operation, yet uses a single integrated VCO circuitry.